@feetwet answer is very good and correct, but maybe the answer to the exact question should be simpler -
In principle itsit's legal to sell software whichthat acts like VNC.
Better then that,: olderOlder versions of the VNC protocol - RFB(RFB) are open source, and the protocol details are well documented -, which would make it hard for anyone to say "your remote viewing software is infringing on our IP"IP."